1. Field of the Invention
This invention relates to a process for producing terephthalic acid by oxidizing para-xylene with molecular oxygen.
2. Description of the Prior Art
Oxidation of para-xylene with molecular oxygen in acetic acid as solvent in the presence of a catalyst system containing bromine and one or more heavy metals such as cobalt, manganese and the like is well known as the S.D. method for the preparation of terephthalic acid. Although this method has various advantages for the commercial production of terephthalic acid, there remains a problem in that loss of acetic acid as the solvent takes place during the reaction and hence the S.D. method involves the defect of enhanced solvent loss. The principal causes for the acetic acid loss are considered to be the combustion of acetic acid and the formation of by-product methyl acetate. Among these, the combustion of acetic acid has already been investigated widely. It has been proposed that the combustion can be controlled by using, for example, specific reaction conditions or a specific catalyst system, and, to a certain degree, good results have been obtained thereby. However, the other principal cause for loss of the solvent, i.e., the formation of by-product methyl acetate has heretofore received little investigation.